Pentagon spokeswoman Cmdr. Sarah Higgins said Mattis "has provided the White House with an updated policy governing the criteria for transfer of individuals to the detention facility at U.S. Naval Station Guantanamo Bay."

“This policy provides our warfighters guidance on nominating detainees for transfer to Guantánamo detention should that person present a continuing, significant threat to the security of the United States," she added in a statement, which did not elaborate on the new policy.

The Pentagon confirmed that it delivered the guidance to the White House after Mattis told reporters on Monday that “right now I'm not working that issue” when asked about the impending deadline.

The order signed by Trump reaffirmed the president's desire to keep the detention facility open by rescinding former President Obama’s executive order to close the facility.

It also said Mattis's guidance should cover how to handle “individuals captured in connection with an armed conflict, including policies governing transfer of individuals to U.S. Naval Station Guantánamo Bay.”

Guantánamo houses 41 men. Obama was unable to fulfill his promise to shutter the facility before leaving office, but did reduce the population by 196 prisoners.

No one has been transferred out since Trump took office.

During the 2016 campaign, Trump promised to “load” Guantánamo “up with bad dudes,” though no one new has been sent there since he took office last year.

The issue of how to handle new detainees has taken on new urgency as U.S.-backed forces in Syria hold hundreds of foreign nationals accused of fighting for the Islamic State in Iraq and Syria (ISIS).

Sending an ISIS detainee to Guantánamo could put the United States on shaky legal ground, experts have said. That’s because the war authorization the U.S. uses to justify indefinite military detention does not explicitly mention ISIS. A detainee would likely immediately challenge their detention in court on that ground, experts say.

Mattis has said the U.S. preference is for the individuals being held by the Syrian Democratic Forces (SDF) to face justice in their home countries, but has declined to comment on whether Guantánamo is an option should that not happen.

Asked about the issue Monday, Mattis reiterated that the U.S. is working with foreign fighters’ home countries to get them take them back, but said it’s complicated in some cases.

“Now, in some cases, those countries have stripped them of their citizenship, so they have a different view as far as what their status is today,” Mattis told reporters. “So this is not simple. Now, that's being worked principally, of course, by State Department, and we're giving all the support that we can. But so far they're all still in SDF custody at this date.”

Mattis also defended the conditions at Guantánamo.

“I am absolutely certain that there is not one thing going on down there that would not be in accordance with the international protocol, the Geneva protocol,” he said.